The effect of Mo deposition conditions on defect formation and device performance for CIGS solar cells

We have studied the effect of native defects on CIGS solar cells on an ongoing basis and recently investigated the role of Na. We modulated access of Na to the growing film by use of Si₃N₄ blocking layers as well as by varying the Mo thickness and density. Our process is tuned to the level of Na coming from our soda lime glass substrates, however, the role of Na is complex. Collectively the results suggest that one role of Na is that of a catalytic agent for the oxidation of V_Se, which is a donor-like defect, to acceptor-like O_Se. In simulating the effect of this defect in AMPS we find that its biggest effect on performance is its presence in the junction interface region, and its presence there gives rise to the J_SC-V_OC trade-off that we observe in our experimental data. This results in the efficiency being pinned in the 13-14% range for our deposition process and explains the difficulties we have had in overcoming this mechanism. These insights also indicate the path to overcoming this performance limitation.